1. Field of the Invention
The present invention is related to an optical pickup which reads information written on an optical disk using a laser diode, a manufacturing method thereof, and an optical disk system equipped with such optical pickup.
2. Description of the Related Art
In a DVD, MO, CD-R/RW, or DVD-R/RW/RAM system or the like which reads information written on an optical disk, a laser diode is used as the light source of an optical pickup. FIG. 9 shows the rough structure of an optical pickup 31 for reading information recorded on an optical disk 36. The optical pickup 31 is equipped with a laser diode 32, a half mirror 33, a collimator lens 34, an objective lens 35 and a light detector 37. The emitted light from the laser diode 32 is reflected by the half mirror 33 and formed into a parallel beam of light by the collimating lens 34. The parallel beam of light is focused on the optical disk 36 by the objective lens 35. The reflected light from the optical disk 36 passes through the objective lens 35 and the collimating lens 34, and is detected by the light detector 37 after being transmitted through the half mirror 33. The half mirror 33 transmits about 50% of the incident light, and reflects about 50% of the incident light. For this reason, one portion of the reflected light from the optical disk returns to the laser diode without being directed to the primary light detector 37. This is called feed-back. The feed-back to the laser diode 32 from the optical disk interferes with the light emission of the laser diode 32, and this generates feed-back noise in the emitted light of the laser diode 32.
A measurement system for measuring feed-back noise is shown in FIG. 1. In FIG. 1, the measurement system includes a laser diode 11, a collimator lens 12, a half mirror 13, an attenuator 14, a total reflection mirror 15, an objective lens 16, a light detector 17 and a noise measuring device 18. The emitted light from the laser diode 11 is formed into a parallel beam of light by the collimator lens 12, and then separated into two parallel beams of light by the half mirror 13. One separated parallel beam of light receives a required attenuation by the attenuator 14, and returns to the laser diode 11 after being reflected by the total reflection mirror 15. The other separated parallel beam of light is focused by the objective lens 16 and received by the light detector 17. The amount of noise in the received emitted light from the laser diode 11 is measured by the noise measuring device 18. In this measuring system, it is possible to measure the effect of the amount of feed-back with respect to the feed-back noise by adjusting the amount of attenuation of the attenuator 14.
In order to prevent feed-back noise, a method of superimposing a high-frequency electric current on the direct electric current driving the laser diode has been proposed (Electronics Letters, vol. 20, No. 20, pp. 821-822). FIG. 2 shows the emitted light at the time a high-frequency electric current is superimposed on the direct electric current when the laser diode is driven. In FIG. 2, when a sinusoidal high-frequency electric current is superimposed on the direct electric current serving as a driving current, the emitted light forms pulses. This proposal uses the phenomenon of multi-mode light emission carried out at the times when the emission of light is begun again after a high-frequency electric current momentarily forces the direct electric current below the threshold value of the laser diode when the high-frequency electric current is superimposed on the direct electric current when the laser diode is driven.
In this proposal, the distance from the laser diode to a reflector is 100 mm, and a 1 GHz high-frequency electric current is superimposed. In this case, the laser diode repeatedly carries out a pulse light emission operation in which light is emitted for a period of 500 psec, and then the emission of light is turned off for a period of 500 psec. Because light will travel approximately 150 mm in 500 psec, in a measurement system having a round trip distance of 200 mm from the laser diode to the reflector, during the state where the emission of light from the laser diode is turned off, the emitted light from the previous light emitting state is reflected by the reflector and forms feed-back (feed-back A in FIG. 2). This feed-back returns during the time from the light emission turning off state to the next light emitting state. When the laser diode repeatedly carries out a pulse light emission operation by alternating between a light emitting state and a light emission turning off state, there is no coherence correlation between the emitted light from the laser diode at the time of the previous light emitting state and the emitted light from the laser diode at the time of the next light emitting state. As a result, because there is no coherent interaction between the feed-back and the emitted light of the next light emitting state, the feed-back noise can be suppressed to a relatively low level.
In accordance with the miniaturization of the latest optical pickups, the distance between the end surface of the laser diode of the optical pickup and the optical disk has been shortened to about 30 mm. Further, the frequency of the superimposing high-frequency electric current has been kept at 300˜500 MHz in order to prevent unwanted radiation. For this reason, because the emitted light from the laser diode during the period of the light emitting state is reflected by the optical disk and forms feed-back (feed-back B in FIG. 2), such feed-back interferes with the light emission from the laser diode, and this generates a large amount of feed-back noise.
Further, a self-pulsation laser diode has also been developed in order to reduce the feed-back noise. This laser diode carries out a pulse light emission operation in which when the laser diode begins emitting light, the absorption coefficient and the like at the emission light wavelength are changed, and the emission of light is turned off, and when the emission of light is turned off, the absorption coefficient and the like return, and the laser diode begins emitting light again. From the fact that the self-pulsation laser diode also carries out multi-mode oscillation when the emission of light is begun, it is difficult for feed-back noise to be generated. However, because even the self-pulsation laser diode has a limited pulsation frequency, when the emitted light from the laser diode during the period of the light emitting state is reflected and forms feed-back in the miniaturized optical pickup, such feed-back interferes with the light emission from the laser diode, and there is still the problem of a large amount of feed-back noise being generated in the emitted light of the laser diode.
In the structures of older related art optical pickups, there is a little influence against feed-back in the multi-mode oscillation laser diode, and in the miniaturized optical pickup described above, because the laser diode receives an effect from the feed-back, there has been a need to evaluate the feed-back noise. The effect of superimposing a high-frequency electric current and the effect of self-pulsation can ultimately be evaluated by the noise in the emitted light of the laser diode, but the measurement of noise is extremely complicated, and it is difficult to evaluate the relationship between the amplitude of the superimposed high-frequency electric current and the feed-back noise in a measurement system for evaluating noise due to feedback.
On the other hand, many methods of evaluating the coherence of the emitted light from the laser diode with a Michelson interferometer have been reported. FIG. 3 shows a structure according to the principle of a Michelson interferometer. In FIG. 3, there is a movable mirror 19 and a fixed mirror 20. The emitted light from the laser diode 11 is formed into a parallel beam of light by the collimator lens 12, and then separated into two parallel beams of light by the half mirror 13. One separated parallel beam of light is reflected by the fixed mirror 20, passes through the half mirror 13, and is detected by the light detector 17 via the objective lens 16. The other separated parallel beam of light is reflected by the moving mirror 19, reflected by the half mirror 13, and then detected by the light detector 17 via the objective lens 16. By moving the movable mirror 19, it is possible to change the difference in the distances of the two separated parallel beams of light to the light detector 17. The coherence of the emitted light from the laser diode 11 is measured from the created difference in distance of the movable mirror 19 and the light electric power detected by the light detector 17.
In current Michelson interferometers, measurements can be carried out easily within the range of relatively small differences in distance, but when a considerable difference in distance is formed in the distance from the front facet of the laser diode of the optical pickup to the optical disk, the range in which measurements can be carried out easily is exceeded.